In the unidirectional solidification or other casting of ingots of silicon and other materials, serious problems of cracking of the ingot result when the ingot forms a tenacious bond with the walls of the silica crucible and then contracts more rapidly during cooling than does the silica (due to mismatch of coefficients of thermal expansion). The ingot goes into tension and the crucible wall into compression; and the resulting stresses in the ingot are sufficient to crack it.
To some extent, the cracking, at least with the silicon ingots, has been solved by adding beryllium or aluminum to the silicon, or by using a crucible with a very thin (typically less than 0.5 mm thick) wall. The former technique, however, is not acceptable when high purity silicon ingots are required, as in semiconductor and photovoltaic applications; the latter is used only to cast small ingots.